1. Field of the Invention (Technical Field)
The present invention relates to a method and apparatus for management of the display of image data, specifically where image data is "overlayed" on other image data. The invention is independent of the video display hardware used to display the overlayed images.
2. Background Art
There are primarily two methods in the art for display of overlayed image data. Both methods are based upon the bit plane concept of imaging. In this technique, digital images have variable two-dimensional matrix sizes generally extending from 64.times.64 to 2048.times.2048 or higher number of pixels per bit plane of resolution. This two-dimensional matrix can be extended in three-dimensional form by the addition of bit planes. Thus, the depth of resolution is determined by the number of bits per pixel which extends the matrix in a three-dimensional form. The greater the depth of resolution, i.e., the number of bit planes, the greater the capability of imaging overlay. The two principal methods of realizing overlays are as follows:
1. Hardware Overlay Technique. In this regard and by example, the bit plane overlay technique is somewhat analogous to stacked transparent cellophane planes upon which different data or images can be written, imprinted or simulated, such as shown in FIGS. 4 and 5. The information on each of these planes are overlayed one to another, and therefore, viewed as a composite picture on the video monitor. Such bit planes can be manipulated independently of each other in what is stated in the art as a non-destructive manner, i.e., by the use of such overlaying methods data is not overwritten in the same plane. In addition, the greater the number of bit plane overlay capabilities, the greater the range of color and color simulation available.
In the development of this art, image manipulation systems have successively added more bit plane capabilities by means of additional display and processing hardware. Certain manufacturers of such systems, for example, Silicon Graphics Computer Systems, have as many as 64 bit planes or bits per pixel as part of their image processing capabilities. The method for such hardware design is one of redundancy in which successive hardware image processing memory boards are added to the system to obtain increased image overlay capability. This method of providing any required number of bit planes (bits per pixel) is expensive and hardware dependent. Software portability between similar but not identical hardware designs for imaging is difficult and expensive.
2. Software Overlay Technique. In this prior art method, a temporary technique is sometimes used to overlay one image upon another, such as a cursor over a background image, and then to remove the cursor from the screen display. One method in which to remove the cursor requires re-display of the entire display without the cursor. This is necessary to avoid the "Pacman effect", i.e., a temporary shadow of the removed overlay. In another method, a copy is made of the pixel values that will be changed when the temporary image is displayed, so that the original pixel values can be restored from the copy upon removal of the overlay. This has been done without the use of additional hardware bit planes, but only on a very limited basis.
In the field of computer graphics, the use of color is important, not only for its enhanced clarity but also to distinguish data in one overlay plane from another. Hue and opacity may be used to give "trueness" to a color and to create a visual sense of transparency. This capability in the art has been achieved primarily by hardware design wherein hue or opacity is varied by the use of an RGB display of primary colors with at least 8 bits per color, therefore, such systems require a minimum of 24 bits per pixel in order to create such true color capability. An example would be the Silicon Graphics, Inc., Iris.TM. computer system for true color which has 64 bits per pixel. This capability of true color has not been a developed feature of software imaging display programs.